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654

Automatic Expansion-Gear.

learn that very little economy was thus secured. A £ gridiron '
valve was adopted for the expansion valve, the ports being split
into eight or nine portions, for'reasons to be explained in the
next paragraph. Clearance is decreased as much as possible in
the back-cut-off valve, especially in Fig. 648, though it must
always be greater than in a single valve.

A Double-ported Valve, as in Fig. 647, is usually adopted for
low-pressure cylinders of marine engines. As frictional loss
depends directly on distance travelled (total pressure being
equal), it is advisable to decrease the travel as much as possible.
This may be'done by dividing the steam ports into two parts,*
as at AB; only half, travel is then required. Of course the valve
must be made somewhat larger, which increases the total pressure,
and consequently the force of friction; so a portion of the back
is often shielded or ' relieved' from pressure by the ring c D,
which lies in the annular groove E F, being kept steam tight with
the back of the valve by the springs GH, and with the groove by
the ring j.

At Fig. 648-is shewn a back-cut-off valve with double ports, the
main valve being designed to shorten the steam ports and decrease
clearance. Fig. 649 shews double port?, both for Meyer and main
valve, the arrows indicating the paths of live steam and exhaust.

Automatic Expansion-Gear.—Instead of connecting the
governor sleeve with the throttle valve, as at (a) Fig. 645, it may
be allowed to alter the travel of a back-cut-off valve, with in-
creased economy and direct action. The most common arrange-
ment is shewn generally at Figs. 271 and 272,'pp. 270 and 271,
and in detail at Fig. 261. The expansion eccentric is coupled to
the central pin of the radius link, the latter rocking on a pin at
the upper end. When the governor sleeve M, Fig. 261, rises, it
lifts, by lever u-and link K, an intermediate valve rod. Thus the
height of the governor decides the height of die in radius link,
and therefore the amount of travel on the expansion valve.
Eccentric travel remaining cojptant, when the engine speed
increases and the govew@r sleeve lifts the die nearer the link
fulcrum, the travel of the valve is decreased, cut-off is earlier,
-and less work done. This brings back the speed to the normal.

V                                 * Or more, as in gridiron valve.

T